The present invention relates generally to digital data communication systems, e.g., modem systems. More particularly, the present invention relates to a technique for determining, at a receive modem, the encoding type of a central office codec.
The use of pulse code modulation (PCM) techniques in the context of digital communication systems is generally well known. For example, PCM encoding schemes have been utilized in connection with the public switched telephone network (PSTN) for many years to enable the encoding of voiceband signals. An internationally standardized PCM encoding protocol is set forth in ITU-T Recommendation G.711 (International Telecommunication Union, 1993). Recommendation G.711 contains the xcexc-law encoding scheme, which is utilized in the United States (and other countries), and the A-law encoding scheme, which is utilized in European and other countries. The entire content of ITU-T Recommendation G.711 is incorporated herein by reference.
PCM modems, such as 56 kbps modem systems compliant with ITU-T Recommendation V.90 (International Telecommunication Union, 1998), generally rely on the transmission and receipt of codewords or levels utilized by the xcexc-law or A-law encoding schemes (the entire content of ITU-T Recommendation V.90 is incorporated herein by reference). Due to the use of different standardized encoding schemes, a given end-to-end communication channel may be associated with a transmit modem located in one country and a receive modem located in a different country. Accordingly, the, transmit-receive communication channel may be designated as: (1) xcexc-law to xcexc-law; (2) xcexc-law to A-law; (3) A-law to xcexc-law; (4) A-law to A-law; or (5) a combination of thereof (e.g., where a number of different conversions are utilized).
Section 3.4 of Recommendation G.711 states that:
Digital paths between countries which have adopted different encoding laws should carry signals encoded in accordance with the A-law. Where both countries have adopted the same law, that law should be used on digital paths between them. Any necessary conversion will be done by the countries using the xcexc-law.
Consequently, in some cross-country situations, the transmit modem codec may follow a different encoding law than the central office codec associated with the receive modem. If the actual xcexc-law or A-law encoding levels are to be utilized by the modem system, then the central office codec type must be known to or identified by the receive or transmit modem. If the central office codec type is known or identified, then the receive modem can match its slicer levels with the particular encoding scheme to reduce the quantization noise inherent in the system. If the central office codec is known to the transmit modem, e.g., the server modem in a V.90 system, then it can use such knowledge to better adjust its echo canceler.
Although a transmit modem may transmit an identification signal (indicating its encoding law) to the receive modem, the encoding type of the central office cannot be easily ascertained from the identification signal. Accordingly, current receive modems may simply trust the identification signal and assume that the central office codec type matches the transmit modem codec type. In other words, the receive modem may not make an independent assessment of the central office codec type.
Accordingly, the present invention provides a technique for enabling a receive modem to accurately identify the encoding law utilized by a local central office codec. The codec type detection scheme is based upon the transmission levels utilized by the PSTN, regardless of whether such levels are xcexc-law or A-law levels. The technique enables the receive modem to identify the codec type of the associated central office independent of the codec type utilized by the transmit modem.
The techniques of the present invention may be carried out in one form by a method for identifying an encoding type associated with a central office codec in a digital data communication system. The method, which may be performed by a receive modem, obtains a set of transmission levels generated in accordance with an encoding scheme utilized by the central office codec, selects a plurality of transmission levels from the set of transmission levels, and analyzes the plurality of transmission levels to determine whether characteristics of the plurality of transmission levels correspond to a specific encoding type. In the preferred embodiment, the receive modem detects whether the central office codec follows the xcexc-law or the A-law scheme.